java-21
Java 21 language and runtime patterns for modern, safe code. Trigger: When writing Java 21 code using records, sealed types, or virtual threads.
- Installs
- 326
- Last 24h
- —
- Trending rank
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- Security
- Not audited
Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/gentleman-programming/gentleman-skills --skill java-21- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads java-21 automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
When to Use
Load this skill when:
- Writing Java 21 application or library code
- Designing immutable DTOs or value objects
- Modeling closed hierarchies with sealed types
- Using virtual threads for blocking I/O
Critical Patterns
Pattern 1: Records for immutable data
Use records for DTOs and value objects, validate in compact constructors.
Pattern 2: Sealed types + pattern matching
Use sealed interfaces/classes and switch pattern matching for exhaustiveness.
Pattern 3: Virtual threads for I/O
Use virtual threads to handle blocking I/O without large thread pools.
Code Examples
Example 1: Record with validation
package com.acme.user;
public record Email(String value) {
public Email {
if (value == null || !value.contains("@")) {
throw new IllegalArgumentException("Invalid email");
}
}
}
Example 2: Sealed hierarchy + switch pattern matching
package com.acme.payment;
public sealed interface Payment permits Card, BankTransfer { }
public record Card(String last4) implements Payment { }
public record BankTransfer(String iban) implements Payment { }
public final class PaymentPrinter {
public String describe(Payment payment) {
return switch (payment) {
case Card card -> "card-" + card.last4();
case BankTransfer bank -> "iban-" + bank.iban();
};
}
}
Example 3: Virtual threads for blocking calls
package com.acme.io;
import java.util.concurrent.Executors;
public final class Fetcher {
public void fetchAll(java.util.List<String> urls) throws Exception {
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
for (String url : urls) {
executor.submit(() -> blockingFetch(url));
}
}
}
private void blockingFetch(String url) {
// perform blocking I/O here
}
}
Anti-Patterns
Don't: Use mutable data carriers
// BAD: mutable DTO
public class UserDto {
public String name;
public String email;
}
Don't: Spin up raw platform threads per request
// BAD: expensive and unbounded
new Thread(() -> blockingFetch("https://api" )).start();
Quick Reference
| Task | Pattern |
|---|---|
| Immutable DTOs | Use records with validation |
| Closed hierarchies | Use sealed interfaces + switch |
| Blocking I/O scale | Use virtual threads executor |
Resources
- Java 21 Release Notes: https://www.oracle.com/java/technologies/javase/21-relnote-issues.html
- JEP Index: https://openjdk.org/jeps/0
FAQ
What does the java-21 skill do?
Java 21 language and runtime patterns for modern, safe code. Trigger: When writing Java 21 code using records, sealed types, or virtual threads.
How do I install java-21?
Run npx skills add https://github.com/gentleman-programming/gentleman-skills --skill java-21 in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/java-21/ (personal) or .claude/skills/java-21/ (project).
Which AI agents can use java-21?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is java-21 safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains java-21?
It is published by gentleman-programming in gentleman-programming/gentleman-skills. It has 326 recorded installs.
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